Lithium iron phosphate (LiFePO4), which has an olivine structure, is a material of appreciable interest in the power sector, particularly as a cathode material for lithium batteries. Its main advantages over lithium cobalt oxide lie in its low cost and its low environmental impact.
Lithium iron phosphate has an olivine structure in which the lithium present between the layers may be extracted and transferred to the anode in the charging process, compensated for by the oxidation of the iron, which passes from its oxidation state +2 to +3. This process may be hampered by impurities, and, as a result, there is particular interest in producing lithium iron phosphate that is substantially free of impurities.
The theoretical capacity of the electron extraction process is 170 mAh·g−1; the main limitation of the material is its low conductivity; this problem may be solved by making use, after the synthesis, of a carbon coating.
To solve the problem of the low conductivity, other routes were explored, including reduction of the particle sizes and the production of a uniform particle size distribution, adaptation of the morphology and structure of the particles via low-temperature synthetic processes and selective doping with multivalent cations to increase the intrinsic conductivity. In this context, the synthesis of nanocrystalline materials also assumes relevance.
There is, therefore, a need for novel production processes, which operate at low temperature and which can produce nanocrystalline particles with a narrow particle size distribution.
A full review of the state of the art relating to processes for synthesizing lithium iron phosphate powders is given by Jugovic D. et al in Journal of Power Sources 190 (2009) 538-544, the content of which is incorporated herein by way of reference.
In summary, the main processes for producing lithium iron phosphate comprise:                synthesis by solid-state reaction, in which stoichiometric amounts of iron(II) acetate, ammonium phosphate and lithium carbonate are heated to the decomposition temperature of 300 to 400° C., under an inert atmosphere, and are then calcined at a temperature of 400 to 800° C. in an oven. Via this method, it is, however, not possible to control the size of the particles, which are very large;        solid-state microwave synthesis, in which the precursors in solid state, rather than being calcined, are irradiated with microwave radiation, by virtue of the fact that iron(II) lactate or acetate acts as a radiation absorber;        solution synthesis via hydrothermal/solvothermal, sol-gel or coprecipitation processes.        
When compared with the solid-state reaction processes, the above processes have the advantage of affording better homogeneity and mixing of the starting compounds at the molecular level;                among the solution processes mentioned above, the hydrothermal route is often adopted.        
Tajimi et al. in Solid State Ionics 175 (2004), 287-290 proposed a synthesis in an autoclave by mixing LiOH.H2O, FeSO4.7H2O and H3PO4 in a 3:1:1 ratio in water. The addition of PEG (in excess relative to the lithium with a 2:1 ratio) makes it possible to reduce the sizes to a level of one micron.
Huang et al. in Materials Characterization 61 (2010), 720-725 proposed a synthesis of nanorods with orders of magnitude of 400-500 nm via a hydrothermal route. LiFePO4 is synthesized by reacting iron sulfate, ammonium phosphate and lithium hydroxide (in a 2:1:1 ratio). Sucrose is added to the mixture, which presumably acts as a growth regulator. The reaction is performed at about 220° C. in a Teflon tube for 18 hours.
Recently, Goodenough et al. in J. Am. Chem. Soc. (2011), 133, 2132-2135 proposed a hydrothermal synthesis using ethylene glycol and ethylenediamine as complexing agents. Microparticles composed of an assembly of nanoleaflets with a thickness of 80 nm are produced: the structure has a high surface area. Once coated with carbon, the microparticles achieve a capacity of 120 mAh·g−1.
Aimable et al. in Solid State Ionics 180 (2009), 861-866 proposed a hydrothermal synthesis under supercritical conditions via a continuous synthetic process.
Peng et al. in J. Power Source (2010) DOI:10.1016/j.jpowsour.2010.10.065 proposed a sol-gel synthesis of lithium iron phosphate/carbon in which iron(II) phosphate, LiOH.2H2O, oxalic acid and glucose are dissolved in water and stirred at 90°, and the solution is then heated to 300° under an argon atmosphere; the process includes a final calcination of the crystals at 600° for 6 hours, to produce crystals of submicron size.
In general, the hydrothermal processes are suitable for working at relatively low temperatures, but long reaction times are however required (up to 24 hours).
Further synthetic routes comprise a microemulsion reaction, described by Xu et al. in Materials Chemistry and Physics 105 (2007), 80-85, and pyrolytic synthesis, described by Konarova et al. in Journal of Power Sources 194 (2009) 1029-1035. In this synthesis, lithium acetate, iron(II) chloride and phosphoric acid are mixed together in water in a stoichiometric ratio, the solution is atomized and the drops are conveyed into a chamber heated to about 500°; the crystals are subsequently milled by wet-ball milling, to obtain crystals of the order of a micron. The process therefore requires high temperatures and subsequent processing of the prepared product.